MDL-46921 lib: Add sorting on additional name fields

A thank you to Yishe Choden for creating a patch for this issue.
MDL-46921 lib: Update get_all_user_name_fields() plus unit tests.
MDL-46921 lib: Sorting on additional fields, admin, enrolment pages.
MDL-46921 lib: behat tests for alternate name sorting.
Merge branch 'wip-MDL-46921-master' of git://github.com/abgreeve/moodle
MDL-46284 tool_messageinbound: Fail with warning if handler disabled

If a handler is disabled, the user should be informed of this issue.

This issue is a part of the MDL-47194 Task.
This issue is a part of the MDL-39707 Epic.
Merge branch 'MDL-47194-master-fixes' of git://github.com/andrewnicols/moodle
MDL-46921 User: Add @core_user to new behat test.